assistant: Make scripting a first-class concept instead of a tool (#26338)

This PR makes refactors the scripting functionality to be a first-class
concept of the assistant instead of a generic tool, which will allow us
to build a more customized experience.

- The tool prompt has been slightly tweaked and is now included as a
system message in all conversations. I'm getting decent results, but now
that it isn't in the tools framework, it will probably require more
refining.

- The model will now include an `<eval ...>` tag at the end of the
message with the script. We parse this tag incrementally as it streams
in so that we can indicate that we are generating a script before we see
the closing `</eval>` tag. Later, this will help us interpret the script
as it arrives also.

- Threads now hold a `ScriptSession` entity which manages the state of
all scripts (from parsing to exited) in a centralized way, and will
later collect all script operations so they can be displayed in the UI.

- `script_tool` has been renamed to `assistant_scripting` 

- Script source now opens in a regular read-only buffer  

Note: We still need to handle persistence properly

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

Cargo.lock

@@ -450,6 +450,7 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "assistant_context_editor",
+ "assistant_scripting",
  "assistant_settings",
  "assistant_slash_command",
  "assistant_tool",
@@ -563,6 +564,25 @@ dependencies = [
  "workspace",
 ]
 
+[[package]]
+name = "assistant_scripting"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "collections",
+ "futures 0.3.31",
+ "gpui",
+ "mlua",
+ "parking_lot",
+ "project",
+ "rand 0.8.5",
+ "regex",
+ "serde",
+ "serde_json",
+ "settings",
+ "util",
+]
+
 [[package]]
 name = "assistant_settings"
 version = "0.1.0"
@@ -11910,26 +11930,6 @@ version = "1.0.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a3cf7c11c38cb994f3d40e8a8cde3bbd1f72a435e4c49e85d6553d8312306152"
 
-[[package]]
-name = "scripting_tool"
-version = "0.1.0"
-dependencies = [
- "anyhow",
- "assistant_tool",
- "collections",
- "futures 0.3.31",
- "gpui",
- "mlua",
- "parking_lot",
- "project",
- "regex",
- "schemars",
- "serde",
- "serde_json",
- "settings",
- "util",
-]
-
 [[package]]
 name = "scrypt"
 version = "0.11.0"
@@ -16984,7 +16984,6 @@ dependencies = [
  "repl",
  "reqwest_client",
  "rope",
- "scripting_tool",
  "search",
  "serde",
  "serde_json",